The Polish–Lithuanian Commonwealth, formally the Kingdom of Poland and the Grand Duchy of Lithuania, (also known as the Kingdom of Poland, or just Poland) was a dualistic state, a bi-confederation, of Poland and Lithuania ruled by a common monarch, who was both the king of Poland and the grand duke of Lithuania. It was one of the largest and one of the most populous countries of 16th- and 17th-century Europe, with some 450,000 square miles (1,200,000 km²) and a multi-ethnic population of 11 million at its peak in the early 17th century.